Define Power Spectral Density Function

Form:  Define Power Spectral Density Functions

Use the Define menu > Functions > Power Spectral Density Functions command to access the Define Power Spectral Density Functions form and add, modify, or delete power spectral density functions. A power spectral density function is a list of function values versus frequency of excitation. The function values in a power-spectral-density function may refer to normalized ground acceleration values or to multipliers for specified (force or displacement) load patterns.

This is a probabilistic type of function. Function values are specified as the square of the input value per unit of frequency. In other words, this function should be specified so that the square-root of the integral of the function over the frequency range gives the expected RMS (root-mean-square) magnitude of loading. Function values may not be negative.

Note: By default, the program pre-defines a unit constant power spectral density function. This function may be modified or deleted.

Important: You may define a power-spectral-density function for any frequency range you want, f0 to fn. For all frequencies less than the first given frequency f0, the function value is assumed to be constant at the function value specified for f0. For all frequencies greater than the last given frequency fn, the function value is assumed to be constant at the function value specified for fn. If you want the function value to be zero outside of a particular frequency range, you must define the first and/or last function values to be zero.

Tip: The frequency range may be specified in Hz (cycles per second) or RPM (cycles per minute). Internally, the program always uses Hz, and results are reported in Hz. The frequency unit also affects the function values, since they are specified per frequency.

Use the buttons on the  Define Power Spectral Density Functions form to add, modify, or delete power spectral density functions as follows:

• Add New Function button.

• 1. Select Function from File or User Function from the Choose Function Type to Add drop-down list.

  2. Click the Add Function button to display the Power Spectral Density Function Definition form  (Function from File or User Function ). The appearance of, and options available on, the form depend on the option selected in Step 1.

• Modify/Show Function button.

• 1. Highlight a function definition in the Functions display area.

  2. Click the Modify/Show button to access the form for a Function from File or a User Function, depending on the type of function selected in Step 1. Note that the form for a Function from File has a button that can be used to change the function to a User Defined function.

• Delete Function button.

• 1. Highlight a function definition in the Functions display area.

  2. Click the Delete Function button to delete the selected function.  A function being used by an load case can not be deleted.